A question came up on exhaust duct connections. I have always considered screws necessary for mechanical fastening of one duct to another. I have been reminded recently that screws impede airflow and catch particles and fibers, all of which is probably true. My source then said that, in their opinion, all ducts should be joined solely by either tape or mastic, or a combination of the two. Is this common practice? Does this increase the time of installation? Does it hold up over time, especially in horizontal runs?

I would appreciate any advice you have.

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I often suggest pop-rivets as an alternative to sheet metal screws for both clothes dryer duct work and for gutters and downspouts, for similar reasons.  The obvious downsides include the need to drill a hole in addition to installing a fastener (slower installation and additional tools), and more difficult disassembly (pop-rivets must be drilled out).  It is good news IMHO that the codes require metal duct work and prohibit the use of screws.  Thanks for that research David.

one maker of duct fans kits puts in #6X 3/8" with 1/4" hex head metal screws.  If you over crimp the 4" pipe will not start the 3/8" long screw.  google" How to prevent fires in  clothes dryers"  I have seen 20+ fires from dryers in 45 yr.  they flash fire with extra air.  Dryer vent need to be cleaned each 6 mo - so how are you going to take it apart to clean?

www.consumerreports.org/clothes-dryer/how-to-prevent-dryer-fires/

www.electrical-forensics.com/Dryer/ElectricClothesDryers

See my reply to Don Fugler earlier. Dryer ducts can usually be cleaned effectivly with a shop-vac and the right connector. The difficulty is getting access to the duct. This is why we recommend this product http://www.fantech.net/enca/home/Products/fans--accessories/accesso... mostly because it gives a good access point for cleaning the duct.

I hear what you are all saying about dryer vents, and how screws there should be avoided (as long as there is some way to clean the duct). Let's move the discussion to kitchen exhaust fan vents. What I gather from this conversation is that the best method would be to pop rivet the sections together, even though this is not a common practice. Then you get the benefits of mechanical fastening without the penetration of screws. I presume that there would have to be screws at some disassembly point for cleaning, unless there is a proprietary duct joining system that works with the lengths being fastened. 

Comments?

The best connector would be what is called generically a "Draw-Band". The Hart & Cooley Catalogue calls theirs the "QC" and it is shown on page 12 of their accessory catalogue (attached). Fantech also has these. These are fairly easy to make up for a sheet metal worker so that is why they are not so popular as purchased items. We use them when we need to get access to a certain component or into a duct easily in the future. An easy way to make them up is to get a length or round duct one or two sizes larger than the one you want to join, cut off a 4" strip, cut-off the snap-lock, wrap the piece you just made around the joint and secure with two suitably sized gear-clamps. We used them at the CCHT when we were installed flow-stations in all of the branch ducts for the zoned HVAC study.

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I agree with Dara's approach, I haven't used the Hart and Cooley QC clamp, nor have I fabricated my own, but I have had success with the Fantech FC Series Clamps.     

The Fantech clamps have the advantage of a rubber liner that also dampens vibration and noise.

I use 5" worm gear clamps  for 4" steel pipe takes a 5/16 nut driver, very easy to clean.  I use a long sweep 4"  28 gage elbow and and use tin snips cut back 3/4'  6 lines then clamp between dryer and  pipe so its easy to clean.

 

You guys have been really helpful. One more question if you have the time ...

If you are hanging horizontal (mostly horizontal) metal exhaust duct runs, is there a specification for the number or location of duct hangers?

Thanks.

That would be in the SMACNA installation standards, but I will not be in the office until Monday & I do not have an E-copy that I can access.

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